1. Field of the Invention
The present invention concerns a device for braking a rotary element comprising at least two distinct braking assemblies each having at least one friction surface or track and an associated friction lining between which is applied, in the contact zone or region during braking, a friction coefficient.
2. Summary of the Prior Art
Disk brakes that have a much more constant and stable friction coefficient than drum brakes but which require greater actuating powers, are now used for almost all vehicles adapted to run at high speed on a track or running path: tourist and sports cars, motorcycles, rapid rail vehicles, regular passenger and freight aircrafts and military planes, etc. The only drawback inherent in such brakes resides in their smaller capacity as parking brake, which thereby still provisionally prevents them from being used for heavy industrial vehicles where the parking brake has an important role.
For more rapid vehicles such as competition cars and motorcycles that exceed speeds of 300 km/h and for the high-speed train (TGV) which also exceeds this speed in trial runs, it has been necessary to use novel materials for disk brakes and for the friction linings cooperating with these disks. Competition cars and motorcycles have thus been fitted with disk brakes made of carbon fibers cooperating with friction linings also made of carbon fibers, while high-speed trains will be fitted with disks made of stainless steel cooperating with friction linings made of sintered studs or pins, while awaiting use at a later date of disk brakes also based on carbon fibers.
One drawback of the brake disks made of carbon fibers which has recently been observed, and in particular on high speed motorcycles, and which has already become apparent to a lesser degree on underground railway (sub-way) cars fitted with wooden brake blocks (hard wood such as ash wood) rubbing on steel wheels, is due to the fact that the friction coefficient of such friction assemblies or pairs (carbon/carbon or wood/steel) is only satisfactory when the friction surface or track is hot. This friction property rather goes against those most currently used up to now (the earliest used friction assembly: cast iron shoe on steel wheel, exhibits a friction coefficient which considerably decreases when the friction speed and the temperature increase, which thereby prevents destructive heating of the wheel and ensures excellent parking braking) and does not allow the rapid establishment of the maximal permitted braking deceleration. This latter requirement which is apparently of little importance for airplanes that brake progressively, has revealed to be very important for other applications, especially for motorcycles where the load transfer onto the front wheel greatly affects the braking control and depends mainly upon the initial deceleration obtained.
One of the aims of the present invention is specifically to overcome these difficulties and to provide that significative braking forces are established, upon application of the brake, allowing the driver of the vehicle to regulate and adjust the sequel or continuation of the braking while enjoying correct feedback information and a good proportionality between the brake actuating forces and the effectively obtained decelerations.
For this purpose, the materials of the two elements of the first assembly being such that the friction coefficient decreases when the temperature of the friction track increases and the materials of the two elements of the second assembly being such that the friction coefficient increases when the temperature of the friction track increases and the friction surface of track of each braking assembly being preferably carried by or formed onto a brake disk, according to the invention, the distinct members for pressing the friction linings are connected to distinct braking pressure generators, for example master cylinders, that have a ratio between their actuating force by an actuating member and the generated pressure which is such that for small actuating forces, only the pressing member of the first assembly is actuated up to a transition point of the actuating force, from which the pressing members of the first and of the second assemblies are simultaneously actuated.
According to another embodiment allowing to further limit the deceleration jerks and jolts provoked by the sudden increase of the friction coefficient of the second assembly, the actuating member of the second assembly cooperates with at least one overbalance spring adapted to reduce the braking pressure of the second assembly with respect to that of the first assembly by a substantially constant value.
According to one advantageous embodiment of the invention, the friction path of the first assembly is made of a ferrous material such as cast iron or steel and cooperates with friction linings made of composite material (for example, asbestos- or plastic material based), giving a friction coefficient that is stable but which rapidly decreases for high temperatures of the friction track, while the friction track of the second assembly is made of a fibrous material such as carbon fibers and cooperates with a friction lining made of a fibrous material of the same type and the friction coefficient of which increases when the temperature of the friction track increases.
According to another embodiment of the invention, giving yet better performances, the friction track of the first assembly is made of a ferrous material having high heat resistance such as a stainless steel in permanent austenitic structure and cooperates with a friction lining made of hard material such as sintered material, while the friction track of the second assembly is made of a fibrous material such as carbon fibers and cooperates with a friction lining made of a friction material of the same type giving a friction coefficient that increases when the temperature of the friction track increases.
According to another very compact embodiment of the invention, the friction tracks of the first and second assembly are formed on two disks, respectively made of ferrous material and fibrous material, which are driven in simultaneous rotation by a group of splines borne by a support drum integral in rotation with the rotary member and are pressed against each other through the intermediary of a plane annular face inside a single caliper or yoke or a single braking clamp adapted to simultaneously press one annular face of the disk made of ferrous material and one annular face of the disk made of fibrous material, so that the two disks are in mutual thermal contact condition. The disk made of ferrous material is advantageously placed in the position enjoying the greatest cooling, i.e. generally, in the position the most exposed to the driving wind of the vehicle, so as to facilitate retaining the heating of the disk made of fibrous material while limiting the drop of the friction coefficient of the other disk made of ferrous material. The mounted-on disks made of different materials can be disposed in two pairs, each pressed by a caliper or yoke, in order to obtain double disks and double calipers brakes comprising, in fact, four brake disks.
According to one embodiment improving the stability of the braked vehicle, in particular a motorcycle equipped with a powerful disk brake on the front wheel, the friction track (or the annular abuting sides) of the disk of the first braking assembly is (are) located on the median plane of the braked wheel, whereby the driving shifts provoked by the initiation of the braking process, which generally starts on this disk which brakes well when cold, are reduced.
In the embodiment of the invention, and if use is made for the first braking assembly of a brake disk made of cast iron and conventional friction linings for automotive and motorcycles, the friction coefficient decreases slowly when the temperature of the disk, and thus of the contact zone, increases up to a temperature known as transition temperature generally situated between 350.degree. and 450.degree. C. and for which the friction coefficient drops rapidly to about half its initial value. When the second braking assembly according to the invention is not foreseen, the user has the impression that the brakes "collapse" without it being able to reestablish the correct level of the braking forces. In the device according to the invention, the second braking assembly has heated and takes over the braking which practically does not decrease although its initial application is able to be both rapid and moderable due to the good progressivity of the conventional assembly: cast iron or steel disk-composite friction linings.
The two disks of the two distinct assemblies have substantially the same external and internal diameters, but different thicknesses and generally, the disk of the second assembly has the greater thickness, which enables to compensate the fact that it dissipates the greatest part of the braking energy and that it is made of a relatively mild material.
According to another embodiment aimed at encouraging the rapid obtention of high braking powers, the disk of the second assembly has a small thickness of 3-4 millimeters adapted to allow its rapid heating during braking.
According to another embodiment allowing to better use the different braking and heat dissipation capacities of the two disks, the distinct members for pressing the friction linings include application pistons having different diameters, the piston(s) of the second assembly having the smaller diameter.